The present invention relates to the removal of gases such as CO2 from analyte streams in liquid chromatography.
The process of suppression as currently practiced minimizes the conductivity of the eluent or mobile phase while maximizing the conductivity of most analytes. This technique is described in several patents (U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019; 3,926,559, 4,474,664, 4,999,098).
In HPLC applications with electrochemical detection the presence of oxygen leads to higher background current and noise. Reim in Anal. Chem, 55, 1983, 1188-1191, showed that the oxygen background can be minimized with the use of gas permeable tubing. The tubings tested for oxygen gas permeability were silicone rubber, Teflon®, 4-methyl-1-pentene, Tygon® and Nafion®. Silicone rubber tubing was chosen because of its higher permeability. Evacuating the outside of the tubing was more effective in oxygen removal versus purging with an inert gas or flushing the outside with an alkaline sulfite sweep solution. The other tubings used in the above investigation had lower permeability to CO2 along with the limitation that the CO2 had to traverse through the tubing wall material. The silicone rubber tubings used in the above work however were fragile. Therefore, there is a need for a pressure stable gas permeable membrane that would be useful for removing O2 from the eluent.
In suppressed IC with carbonate eluents, it was recognized early by some workers that removing the CO2 from a carbonic acid suppressed eluent would allow the implementation of gradients using carbonate and/or bicarbonate eluents. The removal of CO2 led to lower background along with other benefits such as reduction of the water dip or void peak, better integration of the early eluting peaks from the void, lower noise because the background is lower, and higher sensitivity depending on the background. However, the tubings used in the above work were fragile and in some cases had pinholes allowing liquid transport across the tubing walls. Therefore there is a need for pressure stable gas permeable membranes that would be useful for removing CO2 from the suppressed eluent without allowing bulk liquid flow.
During suppressed IC analysis, the peak constituting dissolved CO2 in the sample is detected as suppressed carbonic acid. In some samples this carbonate peak appears as a relatively broad tailing peak and depending on the concentration can interfere with the identification and quantitation of anions that elute in the general vicinity during elution with hydroxide or borate eluents. The problem is particularly acute when a large sample volume is injected. Sample degassing by sonication or by bubbling N2 or helium gas are commonly used to minimize the intrusion of CO2 into the sample. The above approaches work best for acidic samples (pH<6.3, pKa of H2CO3) as dissolved CO2 is largely present in the unionized form and would tend to outgas easily. In alkaline (basic) samples, however the dissolved CO2 is largely present in the ionized form (as bicarbonate and carbonate) anion and cannot be easily removed by degassing approaches. There is a need therefore for a simple online means of removing CO2 from samples for ion chromatography. The role of the prior art gas permeable modules in IC was to remove carbon dioxide from carbonate eluents after suppression. With carbonate eluents, the presence of relatively higher levels of dissolved carbon dioxide in the sample is usually obscured by the high CO2 background from the suppressed eluent (carbonic acid). However in some samples the presence of high concentrations of dissolved CO2/carbonate may still cause problems with the analysis.
U.S. Pat. No. 5,439,736 describes fully alkylated polysiloxane polymer deposited from the gas phase on microporous polymeric hollow fibers. The resulting coating is a thin film crosslinked on the outside of the fibers. Plasma polymerization conditions are stated to lead to uniform, pinhole free, highly adherent and ultra thin coatings. In U.S. Pat. No. 5,439,736 the above cited tubings were stated to be useful for gas phase separations.
Sunden et. al., Anal. Chem. 1984, 56, 1085-1089, described the use of porous PTFE tubings (Goretex®) for the purpose of lowering the background conductivity using hydrogen carbonate/carbonate eluents. By inserting a twisted wire into the gas permeable tubing the authors stated they were able to remove about 90% of the carbon dioxide.
Siemer and Johnson, Anal. Chem. 1984, 56, 1033-1034, in 1984 used silicone tubing for carbon dioxide removal from carbonate/bicarbonate eluents. A 0.1 M KOH solution was warmed and used in the exterior of the silicone rubber tubing. almost complete removal of CO2 was stated to be accomplished by warming the or solution to about 79° C.
In general, the above tubings tended to be fragile and did not offer the pressure stability offered by tubings of the present invention. In some of the above tubings, the diffusion length through the wall of the tubing was significantly large.
Shintani and Dasgupta, Anal. Chem. (1987), 802-808, disclosed a bundle of porous polypropylene tubing coated with silicone as post suppressor devices for lowering the background conductivity with carbonate/bicarbonate eluents. The authors concluded that a baseline correction by subtracting the background (run without injection from a standard run) was better than the use of the above gas permeable post suppressor device. The coating method suggested a thick coating density on the outside of the polypropylene fiber. For example, the above publication recommends that the tubing be coated up to 10 times in order to get a pin hole free tubing.
U.S. Pat. No. 6,444,475 described the use of TEFLON AF gas permeable tubing for the function of removing CO2 from the suppressed carbonate and/or bicarbonate eluents. Although the tubing is described to be pressure resilient, it is extremely expensive.
Therefore, there is a need for a pressure resilient, efficient low cost alternative to the foregoing materials.
When using a preconcentration technique in anion analysis the presence of excessive amounts of carbon dioxide/carbonate in the sample stream will affect the performance of the concentrator column, as the concentrator column will concentrate the carbonate ion along with other sample anions of interest. In addition to reducing the effective capacity of the concentrator the presence of carbon dioxide/carbonate in the sample can also impact the capture efficiency of the concentrator column as the carbonate ions tend to elute the other sample ions of interest. The peak shapes can also suffer because the sample plug is diffused in the concentrator due to partial elution and the sample is injected as a broad plug into the separator. Therefore there is a need for removing carbon dioxide/carbonate in sample streams particularly while using a preconcentrator column.
Similarly in cation analysis, presence of high levels of ammonia during the preconcentration step has a deleterious effect as discussed above. Therefore there is a need for removing the interference of ammonia in sample streams.